1. Field of the Invention
The present invention relates to pilot operated hydraulic valves, and more particularly to such valves which incorporate mechanisms that compensate for variation of a pressure differential which exists across a pilot orifice.
2. Description of the Related Art
A wide variety of machines have moveable members which are operated by an hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. Traditionally the hydraulic valve was manually operated by the machine operator. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located near an operator station, but can be located adjacent the actuator being controlled. This change in technology also facilitates computerized control of the machine functions.
Application of pressurized hydraulic fluid from a pump to the actuator can be controlled by a set of proportional solenoid valves of a type described in U.S. Pat. No. 5,878,647. Solenoid operated pilot valves also are well known for controlling the flow of hydraulic fluid and employ an electromagnetic coil which moves an armature in one direction to open a valve. The armature acts on a pilot poppet that controls the flow of fluid through a pilot passage in a main valve poppet. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or another valve member is spring loaded to close the valve when electric current is removed from the solenoid coil.
When an operator desires to move a member on the machine a control lever is operated to send electrical signals to the solenoid valves for the hydraulic actuator, for example a cylinder-piston combination, associated with that machine member. One solenoid valve is opened to supply pressurized fluid to the cylinder chamber one side of the piston and another solenoid valve opens to allow fluid being forced from the opposite cylinder chamber to drain to a reservoir, or tank. By varying the degree to which the solenoid valves are opened, the rate of flow into the associated cylinder chamber can be varied, thereby moving the piston at proportionally different speeds.
Conventional pilot-operated poppet valves are single directional. There is an inlet port and an outlet port of the valve and the pressure at the inlet port is communicated to the pilot control chamber thus enabling the valve to open when the inlet port pressure is greater than the pressure at the outlet port. This enables fluid to flow from the inlet port to the outlet port. Because of this arrangement, the valve can not be used to control the flow of fluid in the reverse direction from the outlet port to the inlet port. In some hydraulic systems, a bidirectional flow is desired to be controlled. To accommodate flow in both direction a second valve connected in a reverse parallel manner to the first valve was required. Therefore, it is desirable to create bidirectional pilot-operated poppet valve.
One type of bidirectional pilot valve has first port that leads to a side of a main poppet and a second port that leads to the nose of the main poppet. Typically the supply line from the pump was connected to the first port. An O-ring seal around the main poppet generally prevents fluid from leaking along the outer surface of the poppet between the first port and a pilot chamber of the valve. However, when the machine was powered down and the pump no longer supplies pressurized hydraulic fluid to the valve. Some leakage occurs past the O-ring seal over time. For example, it is common that over the night or weekends when a construction site is vacant, compressors and other pieces of equipment are stored raised by a front end loader for security reasons. Thus over these prolonged periods of machine non-use, leakage past the valve O-ring seal can result in the stored equipment dropping down. Thus it is desirable to reduce the number of leakage paths through the hydraulic valves.
A bidirectional, pilot-operated control valve has a body with a first port, a second port, and a valve seat between the first and second ports. A main valve poppet selectively engages the valve seat to control flow of fluid between the two ports. A control chamber is formed within the body on a side of the main valve poppet that is remote from the valve seat.
The main valve poppet has a pilot passage which opens into the control chamber through a pilot orifice. A first passage in the main valve poppet provides communication between the first port and the pilot passage, and a second passage creates a fluid path between the second port and the pilot passage. The main valve poppet has a third passage which provides communication between the first port and the control chamber, while a fourth passage forms a fluid path between the second port and the control chamber.
Each of the four passages has a flow control element, such as a check valve, for example, which allows fluid to flow in only one direction through the respective passage. Specifically, a first flow control element in the first passage allows fluid to flow only from the cavity into the first port. A second flow control element in the second passage allows fluid to flow only from the cavity into the second port. A third flow control element in the third passage allows fluid to flow only from the first port into the control chamber. A fourth flow control element in the fourth passage allows fluid to flow only from the second port into the control chamber.
An actuator operates a pilot poppet to selectively open and close the pilot orifice in the main valve poppet. Opening and closing of the pilot orifice produces movement of the main valve poppet with respect to the valve seat and controls the flow of fluid between the first and second ports.
Another aspect of the present invention is a novel mechanism which compensates for the effects of a varying pressure differential across the pilot orifice. This pressure compensating mechanism is associated with the pilot passage of the main valve poppet and is acted on by that varying pressure differentia. In one embodiment, the pressure compensating mechanism includes an end member slidably received in the pilot passage proximate to the control chamber with the pilot orifice extending there through. A spring is formed by a plurality of helices projecting from the first end member through the cavity and engaging the main valve poppet. The spring compresses and expands in response to variation of the pressure differential across the pilot orifice thus moving the pilot orifice to compensate for the effects of the pressure differential variation on the pilot operation of the main valve poppet.